2. Technical information

The central DHCP service runs on two servers, dhcp-b.ox.ac.uk and dhcp-c.ox.ac.uk. The two servers are physically on separate networks within OUCS, giving some degree of redundancy in the event of network problems. Inter-server communications allow the servers to exchange state information for load-balancing and resilience.

All broadcast requests on port 67 within the university are intercepted by the routers, and forwarded to the two central DHCP servers. However, the servers will ignore requests coming from a subnet unless that subnet has been registered to the OUCS DHCP service. Furthermore, no response to a request will be given unless the client MAC address is registered, OR the request comes from a subnet configured to allow any client machine to request an address by DHCP, irrespective of whether it is registered. Thus the central servers should not accidentally interfere with the operation of any DHCP server operated by an individual unit.

In the event that both DHCP servers make a DHCP offer to a requesting client, the client will respond and take an address from the first to reply. The address offered by the second DHCP server will be released and made available to other requesting clients.

The default lease time is four hours, although it is possible to request this to be changed this on a per-subnet basis.

As well as allocating an IP number to a requesting client, the servers also return DNS and WINS server information. If appropriate these can be varied on a per-subnet basis.

Note that DHCP can cause problems on subnets which are "shared" between two or more units (usually college annexes or small departments), leading to the allocation of an address in another unit's namespace.

See also services and documentation for network administrators.

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